1. Field of the Invention
This invention relates to insulation for refrigerators and freezers.
2. Description of the Related Art
Previously, several different materials and material combinations have been suggested in order to increase the heat insulation characteristics for walls and doors in refrigerators and freezers. It has also been suggested to use so called "vacuum panels". Conventional insulations usually comprise foamed polymeric materials, whereas for vacuum panels, an evacuated shell of diffusion-tight material, for instance plastic or sheet metal, which is filled with powder or cellular material is used. This last-mentioned arrangement is described for instance in SE 90937, EP 188806, JP 63135694, U.S. Pat. No. 5,066,437. The arrangement has, however, certain drawbacks since it is difficult to maintain sufficiently low pressure during the complete lifetime of the cabinet, which is 15-20 years, since minor leakage decreases the heat insulation characteristics. Further, it is difficult and expensive to carry on the evacuation process to a desirable extent in mass production since such an evacuation process takes a very long time. Because of the long and narrow evacuation passages, it takes at least 15 hours to reduce the pressure to 1 mbar independently of the capacity of the vacuum pump, whereas the production time for a refrigerator is about 20 minutes.
In order to make it possible to evacuate slightly faster, as appears from some of the above-mentioned publications, polymeric materials with open cell structure have been used. The disadvantage with an open cells structure, both with regard to conventional insulations and vacuum insulations, is, however, that with such a structure, it is difficult to fulfill the demands for mechanical strength at lower densities. In practice, it has been necessary to use comparatively high densities which means that the price, weight, and heat conductivity in the solid state increase considerably.
A closed cell structure in combination with adherence to the surrounding shell gives mechanical stability also at comparatively low densities, but requires small cells in order to minimize the heat transportation by radiation and in order to get superinsulation (which means that the free length of movement of the molecules should be of the same magnitude as the size of the cell) at as high pressure as possible.
The above-mentioned desires regarding closed and open cells are thus contradictory which means that the properties which have been regarded as most important, i.e. mechanical strength or the possibility to evacuate the insulation quickly, have determined what kind of cell structure that should be used.
It is also known, see U.S. Pat. No. 4448041, to use vacuum insulated wall elements for large mobile cold storage rooms, the wall elements communicating with a vacuum pump. These vacuum pumps are, however, of conventional type and hence comparatively power demanding and expensive. Their use can, with regard to costs and energy consumption, only be suggested for the type of large construction which is described in the above-mentioned publications.
Further FR 2628179 describes hermetically sealed wall elements which, in a manner not shown in detail, are connected to some kind of vacuum source, the 50-100 mbar pressure which is created is comparatively high and, being in such an interval, cannot in any higher degree contribute to increase the heat insulation capability.